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The invention relates generally to electronic circuit design and simulation and particularly to a system and method for simulating the noise characteristics of phase locked loops and other devices in transient analysis.
Designers of electronic circuits use a variety of simulation tools to simulate potential new circuit designs. Examples of such circuit design tools include those developed by Antrim Design Systems, Inc., and Cadence Design Systems, Inc. These circuit design tools are particularly important in the area of telecommunication circuit design, and in particular the design of wireless transmitters, since the transmitters must meet stringent requirements for the spectrum of the signal they send out or transmit.
Common electronic devices found in telecommunication circuits include oscillators and phase-locked loops. Generally described, an oscillator is an electronic device that can be used to generate a signal. Oscillators are found in a wide variety of devices and appliances, including computers and cellular telephones. Most oscillators operate by employing a sensitive amplifier, the output of which is fed back to the input. In this way the signal uses positive feedback to sustain itself. The frequency at which an oscillator operates can be determined by a quartz crystal, although a combination of inductors, resistors, and capacitors can also be used to determine the frequency.
A phase-locked loop (often referred to as a PLL) is an electronic circuit with a voltage-driven or current-driven oscillator that is constantly adjusted to track the frequency of an input signal. In the telecommunications field, PLL""s are often used to stabilize or lock onto a particular communications channel. PLL""s are also frequently used in wireless communication systems. A typical PLL comprises a voltage-controlled oscillator (VCO), which is tuned to a frequency close to the desired receiving or transmitting frequency of the PLL. The VCO output signal is typically divided in a frequency divider. A phase comparator allows the output of the divider to match the output of a reference oscillator. If the VCO frequency departs from the desired frequency, the output of the divider will correspondingly depart from the reference frequency, and the phase comparator applies a voltage to bring the VCO back to that desired frequency. The reference oscillator and the PLL, with the VCO, the divider and the comparator may together be considered a frequency synthesizer.
Both oscillators and phase-locked loop devices are nowadays commonly manufactured as integrated circuits instead of as discrete circuits that use discrete inductors, comparators, etc. Because phase noise is very important in a telecommunication circuit, when designing these integrated circuits, designers need tools to predict the circuit""s noise characteristics. Currently, tools do exist for predicting the noise characteristics of a VCO. These include RF simulators such as SpectreRF from Cadence Design Systems. However, these tools can not run a large signal transient analysis on the whole PLL.
With the introduction of mixed mode simulators, which can handle both analog behavioral and digital, the simulation runtimes become reasonable for a transient analysis of a Phase Locked Loop (PLL) in the millisecond range. An embodiment of the present invention allows for Voltage Controlled Oscillator (VCO) noise, and any noise generated in the digital circuitry (by fractional dividers for example), to be included in the transient analysis. Other relevant noise sources may also be included. The phase noise characteristics of the VCO are first determined by a Radio Frequency (RF) simulator. A transient noise analysis is then performed using this data, thus including the VCO phase noise and the noise produced in the digital feedback part of the PLL. The system can also be used with other mixed mode simulators, and even system level simulators such as the Signal Processing Worksystem (SPW).
As described herein, an embodiment of the invention provides a system and method that utilizes behavioral models in simulating the noise characteristics of circuits and devices such as phase locked loops. In accordance with one embodiment, an initial step is to retrieve, or to generate VCO phase noise data for a given circuit. In practice, this VCO phase noise data may be generated by or received from an RF simulator. The phase locked loop (PLL) blocks are converted to calibrated behavioral-level models. This step enables the system to be represented as behavioral-level models instead of as discrete components, and allows the circuit to be simulated at a higher level of abstraction. The PLL simulation is verified using these models, without the noise generator being turned on or implemented, so that the noise floor of the mixed signal simulator can be determined. The transient analysis is then run with the noise generators turned on, and the results of the transient noise analysis post-processed to create noise spectrum data, which is then used to create noise spectrum plots. The noise spectrum data and/or plots may also be used in other ways, such as with other applications or with other analysis software.